1. Field of the Invention
The present invention relates to optical discG recording/reproducing apparatuses, and more particularly, to an optical disc recording/reproducing apparatus capable of recording/reproducing information to/from optical discs having different substrate thicknesses or recording densities.
2. Description of the Background Art
An optical disc reproducing apparatus has been recently provided which reads information recorded on an optical disc having a thickness of approximately 1.2 mm such as a CD (Compact Disc) and a CD-ROM with a semiconductor laser. In such an optical disc reproducing apparatus, focus servo control and tracking servo control are carried out with respect to an objective lens for pickup. A laser beam is directed to a pit train on a recording surface, whereby a signal such as sound, video, and data is reproduced.
In order to record a long motion picture on such an optical disc, a technique of making the recording density high has been progressed in recent years. For example, an SD (Super Density) standard (DVD standard) is proposed which records information for approximately 5 Gbytes on one side of an optical disc having the same diameter as that of a CD-ROM (12 cm). According to the SD standard, the thickness of the optical disc is approximately 0.6 mm. Information for approximately 10 Gbytes can be recorded on one optical disc including two SD specified disc substrates laminated with their signal surfaces therebetween. On the other hand, an MMCD (Multimedia Compact Disc) standard using a one-layered structure is proposed which records information for approximately 3.7 Gbytes on one side of an optical disc having the same diameter as that of the CD-ROM (12 cm). According to the MMCD standard, the thickness of an optical disc is approximately 1.2 mm. Information for approximately 7.4 Gbytes can be recorded on one side of an optical disc according to the MMCD standard using a two-layered structure.
An objective lens for pickup is designed in consideration of the thickness of a substrate of an optical disc to be read and the wavelength of a semiconductor laser to be used. Therefore, an optical disc having the substrate thickness different from the design cannot be read, since the spot of a laser beam is not converged on a recording surface of the optical disc. For example, an objective lens designed to be adapted to an optical disc having a substrate of 1.2 mm in thickness cannot converge the spot of a laser beam on a recording surface of an optical disc having a substrate of 0.6 mm in thickness, and cannot reproduce information recorded on such an optical disc.
Tanaka et al. discloses in Japanese Patent Laying-Open No. 5-303766 an optical head including an aspherical optical element in order to correct aberration caused by the difference in substrate thickness of an optical disc. This optical element may have a function of changing the numerical aperture (NA) of an objective lens.
An objective lens is generally displaced in a direction (tracking direction) perpendicular to the optical axis of a laser beam by tracking control. However, the aperture disclosed by Tanaka et al. is fixed to the optical axis of the laser beam, irrespective of tracking control. Therefore, if the objective lens is displaced in a similar tracking range to that in the case where there is no aperture provided, the deformation of the spot of the laser beam directed to a recording surface increases according to the amount of shift of the optical axis of the objective lens with respect to that of the laser beam. This is because the diameter of the laser beam reduced by the aperture causes a great deformation of the spot of the laser beam, as if the amount of displacement of the objective lens is relatively increased.
Such a beam spot deforms not only in the track direction but also in the tracking direction perpendicular thereto. The deformation of the beam spot in the track direction causes deterioration of jitter. The deformation of the beam spot in the tracking direction causes crosstalk noise. Therefore, an optical disc having a substrate of approximately 1.2 mm in thickness cannot be read stably. Further, an optical disc according to the MMCD standard cannot be read.
In the future, coexistence of an optical disc having the current density and a substrate thickness of approximately 1.2 mm (CD, CD-ROM), an optical disc according to the MMCD standard having a high density and a substrate thickness of approximately 1.2 mm, and an optical disc according to the SD standard having a high density and a substrate thickness of approximately 0.6 mm is expected. The optical discs according to the MMCD and SD standards are referred to as a digital video disc (DVD), in order to be differentiated from the CD and the CD-ROM.
The MMCD and SD are temporary names. They may be changed in the future. In the present application, the MMCD and SD standards are used as determining physical characteristics of an optical disc such as substrate thickness and recording density.